In the normoxic heart, most of the adenosine (AR) formed from AMP is rephosphorylated by AR kinase (AK), some is deaminated by adenosine deaminase (ADA) while little AR is released. Inosine (HR) is also formed from IMP, the product of AMP deaminase, and is degraded to hypoxanthine (HX), xanthine (X) and uric acid (UA). Due to the high activity of the AMP-adenosine metabolic cycle very little AR is released (0.07 nmol/min/g) from the normoxic heart when compared to IR, HX, X and UA (1.1, 0.4, 0.2, 1.4 nmol/min/g.). The role of this metabolic cycle in hypoxia was investigated in isolated guinea pig hearts. Free cytosolic AMP was determined by 31P NMRand coronary venous purine release by HPLC; AK and ADA were selectively blocked by iodotubercidin and EHNA. There was a linear relation between free AMP (200-3000 nmol/L), net AR formation (AMPAR) and HR release during ADA blockade (IMPHR). Surprisingly AR release rose several-fold more than AR formation. Switching to 40% O2 increased free AMP and AR formation 4-fold, while cytosolic AR and AR release rose 20-fold. While at 95% O2 only 6% of AR formed were released, this fraction increased to 22% already at 40% O2 demonstrating reduced AR salvage. Selective enzyme blockade indicated that flux through AK decreased from 85 to 35% of AR formation in hypoxia. Mathematical model analysis demonstrated that this decrease in enzyme activity was due to inhibition of AK activity to 6% of basal levels. The data show a) that AMP substrate concentration directly controls AR formation by 5'-nucleotidase and most likely flux through AMP deaminase and b) tha t hypoxia decreases AK activity, shunting myocardial AR from purine salvage to venous release. Because of the normal high turnover of the AMP-adenosine metabolic cycle, hypoxia-induced inhibition of AK causes the amplification of small changes in free AMP into a major rise in AR. This mechanism plays an important role in the high sensitivity of the cardiac AR system to impaired oxygenation.